Co-planar constant-attenuation phase modifier

ABSTRACT

The invention relates to a phase shifter for high-frequency electric lines ( 36 ), wherein phase shifting is essentially achieved by specifically selecting the line length, the device essentially consisting of a circuit arrangement ( 30 ) equipped with coplanar lines. The adjustment possibilities of the various-length coplanar lines ( 32, 34 ) with regard to ohmic damping and impedance are preselected in such a way that ohmic damping and impedance are essentially the same on the selectively controllable, various-length conductive paths ( 32, 34 ) of the circuit arrangement ( 30 ). Adjustment possibilities include the width w of the particular central conductor ( 24 ) and the width b of the outer conductor ( 22 ), in addition to the spacing g between the central conductor ( 24 ) and the outer conductor ( 22 ). In conforming the various-length lines ( 32, 34 ), the situation that is specific for coplanar lines is utilized, namely that the impedance depends on w and g, but the ohmic resistance depends essentially only on w, that is, these two physical variables are capable of being adjusted quasi independently of each other. Since the ohmic damping and impedance are the same for the various-length conductive paths ( 32; 34 ), a change-over of the phase state is achieved while the insertion loss remains nearly constant. Phase shifters of this nature are suitable for beam sweeping in phased arrays in motor vehicle sensor technology. The radiation characteristics remain the same when the phase is shifted.

BACKGROUND INFORMATION

The present invention is based on devices for phase shifting forhigh-frequency electric lines, wherein phase shifting is essentiallyachieved by specifically selecting the line length.

Phase shifters are devices with which the phase of a signal and/or analternating current for the subsequent locations of a line or otherelectrical devices are shifted in comparison to the state without phaseshifters and/or in comparison to parallel lines. Phase shifters of thisnature are usually switchable, so that at least two phases that areshifted relative to each other are alternately selectable. “Highfrequency”, in the sense of the present application, refers tofrequencies that are suitable for radar or microwave antennae orcommunications technology, whereby frequencies for wavelengths in themillimeter range are covered in particular by the invention.

Switchable phase shifters are used primarily in phased arrays, which arecurrently of great interest in the field of automotive technology.Phased arrays as microwave antenna with electronically steerable orswitchable radiation lobes are preferably considered specifically forthe further development of motor vehicle radar ranging sensors. Possiblefields of applications in the automotive industry include long rangeradar (LLR) for adaptive cruise control (ACC), and short range radar(SRR), e.g., for parking aids, blind zone monitoring and pre-crashairbag release. Furthermore, there is a large number of civil andmilitary applications in the field of radar and communications [1].

In the operation of a phased array 1 of this nature, which is depictedschematically in FIG. 1, the transmit signal from a signal source 3 isfirst divided by power splitter 5 in accordance with a specifiedamplitude distribution into M columns and/or N lines, out of whichphased array 1 is composed. Beam sweeping takes place in the plane (orin both planes) perpendicular to the columns (or lines) of antenna 1 inthat the phases of the signals that are emitted from individual antennaelements 9 are shifted relative to each other using switchable phaseshifters 7.

A large number of concepts for phased arrays with a steerable radiationlobe and for phase shifters is known in the related art. Refer, forexample, to [2], [3], [4] in the list of literature references providedat the end of the present description.

One certain type of phase shifter is the detour phase shifter. Two ormore line sections having different lengths are switched alternatelybetween the input and output of said detour phase shifter, so that thesignal travels from the input to the output via one of the lines. Thedesired phase shift is obtained via the line lengths. For more than twophase states, detour phase shifters are usually cascaded. Variationswith 1-on-4 change-over switches, for example, that switch between fourline sections, are known as well.

There are different possibilities for realizing the change-overswitches. For example, the lines can be short-circuited at a spacing ofone-fourth of a wavelength from the branching. Micro-electromagneticswitches (MEM switches), in particular, are used in the high-frequencyrange, because they have very good high-frequency characteristics. Otherswitches that are suitable for high-frequency signals, such as pindiodes, FETs or HEMTs (high electron mobility transistor), are also usedin phase shifters, however. Refer to [4 Vol. 2].

Reflection phase shifters are another type that is known in the relatedart. With reflection phase shifters, the path of the signal to adirectional coupler or a circulator is changed by switching the lengthof the signal paths up to one or more transition points, thereby varyingthe phase [4 Vol. 2].

“Loaded line” or “stub-loaded line” phase shifters are another type thatis known in the related art [4], [12]. With phase shifters of thisnature, the phase of the signal is varied by influencing the propagationcoefficient of the signal in the line by overriding reactances that areformed, e.g., using different line lengths (“stubs”).

In reflective “loaded line” and “stub-loaded line” phase shifters, thephase shift can also be achieved by switching over between differentreactances, instead of between different line lengths. These reactancescan be formed, e.g., by changing the capacitance of a pin diode or byswitching over a HEMT (high electron mobility transistor) from theoff-state to the on-state. Hybrid forms are possible as well, e.g.,switching a line length while simultaneously utilizing the changingreactance of the switching element. The switching elements should have a(capacitive or inductive) reactance, of which the ohmic portion shouldbe as low as possible, because the ohmic portion results in losses inthe phase shifter.

A general problem with all phase shifters that are based on the conceptthat the signal travels along path having a different length dependingon the desired phase state, as is the case with reflection phaseshifters and detour phase shifters, for example, is that dampingincreases with signal path length.

The amplitude distribution of the signals on the antenna elementstherefore changes, depending on the phase states of the signals, whichresults in the radiation characteristics of the antenna changing. Ingeneral, the suppression of the minor lobes, in particular, worsens.

Since the ohmic losses of pin diodes or HEMTs, for example, differ inthe off-state state and the on-state in phase shifters with switchedreactances, this also results in a variation of the output amplitude ofthe phase shifter with the phase state, even when the line length doesnot change when the phase state is switched.

In “loaded line” phase shifters, the propagation coefficient and,therefore, in general, the line impedance, changes. The line impedance,which changes with the phase state, results in a mismatch that varieswith the phase state and, therefore, in an insertion loss that varieswith the phase state.

The dependence of the insertion loss on the phase state has not yet beenreduced to a satisfactory extent, despite considerable efforts.“Insertion loss” is understood to mean the damping of the signal that isdue to the phase shifters that are inserted in the conductive path. Itessentially depends on the mismatch of the inputs and outputs of thephase shifter, the line losses, and the ohmic losses of the switchingelements.

Although phase shifters with MEM switches using microstrip technology,configured as reflection phase shifters [8] or detour phase shifters[9], exhibt one of the lowest insertion losses known from the applicableliterature, the insertion loss still exhibits a variation ofapproximately 1 dB, depending on the phase state. This value is stilltoo high. As a result, the application of phase shifters of this naturefor phased arrays in sensor technology, in particular, is problematic.

In military radar systems, vector modulators that can modulate thesignal in phase and amplitude are used in beam shaping. This would allowa variation of the insertion loss of the phase modulator to be correctedby the amplitude modulator. In “moderate” cost applications such asmotor vehicle ranging sensors, concepts of this nature are not yetpracticable, however, because they are very cost-intensive.

Further efforts to rectify the damping problem, so far inadequate, arebeing carried out in the field of coplanar technology. Coplanar lineshave become increasingly well-established in high-frequency switches inthe millimeter-wave range. The configuration of said lines 10 isillustrated in FIGS. 2 and 3. Located on a substrate 20 having thicknessd, which said substrate can be composed of numerous layers, are twometallic outer conductors 22 with a metallic central conductor 24located between them. Central conductor 24, which carries the signal,has width w and height tw. Two outer conductors 22 have widths ba andbb, and heights ta and tb. Widths ga and gb of gaps 26 between centralconductor 24 and outer conductors 22 are usually the same, but are notnecessarily so.

The description of a phase shifter that is composed of a “stub-loadedline” phase shifter and a reflection phase shifter with coplanar linesand HEMT switches is provided in [10]. The insertion loss varies byapproximately 5 dB with the phase state, however, which is far outsidethe tolerance range for the application in phased arrays, in particular.

ADVANTAGES OF THE INVENTION

With the device as recited in Claim 1, a change-over of the phase stateis achieved for high-frequency electric lines while the insertion lossremains nearly the same. According to the invention, when the ohmicdamping and impedance in the various-length lines are conformed, thesituation that is specific for coplanar lines is utilized, namely thatthe impedance depends on width w of the central conductor and gap widthg, but the ohmic damping depends essentially only on w, that is, thesetwo physical variables are capable of being adjusted quasi independentlyof each other. Further technical background about this is provided in[5], [6], [7].

Since the ohmic damping and impedance are nearly the same for thevarious-length conductive paths, the insertion loss is nearly the samefor both paths. Phase shifters of this nature are suitable for beamsweeping in phased arrays in motor vehicle sensor technology. Theradiation characteristics remain the same when the phase shifts.

As a result, according to the invention, phase changes for beam sweepingwith amplitude distribution that remains the same are made possible forphased arrays in a cost-effective manner. The radiation characteristicstherefore remain independent of the phase position, and the suppressionof the minor lobes is therefore ensured to remain the same.

Advantageous embodiments, further developments and improvements of theparticular object of the invention are indicated in the subclaims.

According to an advantageous embodiment of the present invention, byadjusting the width w of the central conductors and the spacing g of thecentral conductors from the particular outer conductors, it is possibleto obtain essentially the same impedance and the same ohmic damping forvarious-length coplanar conductive paths. As a result, the insertionloss is nearly independent of the phase state. Even more advantageous isthe possibility of also incorporating the width of the outer conductorsas a variable parameter in the conforming of impedances and ohmicdampings. This expands the range of feasible phase shifts for the casein which the remaining basic conditions, such as the size of the phaseshifter, are fixed.

An advantageous further development according to the invention is theuse of tapers for transitions to other line geometries. A taper is acoplanar line section with changed line geometry, e.g., with regard forw, g and b, but with an unchanged line impedance, whereby thetransitions take place via gradual, quasi flowing changes in the linedimensions. The flowing transitions allow reflectances and emissions tobe avoided. The use of one or more tapers with a tapered centralconductor as the damping element is also an advantage.

Furthermore, conductive bridge connections between the outer conductorsof a coplanar line that extend over or under the central conductor areadvantageous; this applies for the areas of line branchings, inparticular. The interfering second mode is suppressed as a result, asdescribed in [11].

In addition, the ohmic damping can be varied by using inductive linesections with central conductors that are tapered accordingly. Said linesections serve primarily for compensation, with regard for lineimpedance, of the additional capacitance that is brought about by thebridge connections. This is achieved by increasing the inductivity. Thetapering of the central conductors that is useful for this purpose hasthe additional effect of increasing the ohmic damping of the shortercoplanar lines, so that it can therefore be adapted to that of thelonger lines. For purposes of conforming, the capacitance of the bridgeconnections and, therefore, the length of the compensating inductiveline sections can be increased accordingly. A larger number ofstandardized bridge connections or a variation in the width of suchconnections are other advantageous possibilities.

There is a large number of further advantageous embodiments according tothe invention for equalizing the ohmic damping. For example, to name buta few, additional damping material can be provided on the coplanar linesof the shorter conductive paths, the cross section of the centralconductor can be reduced, or material with lower conductivity can beused.

A further advantageous embodiment according to the invention is the useof MEM switches as switching elements, because they have very goodhigh-frequency characteristics, in particular low ohmic damping.

DRAWING

Preferred exemplary embodiments of the present invention are explainedwith reference to the drawing.

FIG. 1 shows a schematic configuration of a phased array with tworadiation lobes that are capable of being steered in two directions,according to the related art;

FIG. 2 is a sketch of the configuration of a coplanar line according tothe related art, shown in a top view;

FIG. 3 is a sketch of the configuration of a coplanar line according tothe related art, shown as a cross section from the front;

FIG. 4 is a basic structure of a detour phase shifter, according to theinvention, in coplanar technology;

FIG. 4 a is a variation of the basic structure of a detour phaseshifter, according to the invention, in coplanar technology;

FIG. 4 b is a further variation of the basic structure of a detour phaseshifter, according to the invention, in coplanar technology;

FIG. 5 is a sketch of a taper for transitioning to a different coplanarline geometry;

FIG. 5 a is a sketch of a variant of a taper for increasing the ohmicdamping;

FIG. 6 is a sketch of a coplanar line with bridge connection, shown incross section from the front;

FIG. 7 is a sketch of a coplanar line section with bridge connection andthe inductive line section that for compensates for the capacitance ofsaid bridge connection, with regard for impedance;

FIG. 8 is a view of a line branching with connection bridges in anembodiment, according to the invention, of a detour phase shifter incoplanar technology.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the figures, the same reference numerals refer to the same orfunctionally-equivalent components.

FIG. 4 is a sketch of the basic structure of a detour phase shifter 30,according to the invention, in coplanar technology. FIGS. 4 a and 4 bare variations of embodiments, according to the invention, of a detourphase shifter 30 of this nature.

Detour phase shifter 30 contains a coplanar line 32 with a shortconductive path, and a coplanar line 34 with a long conductive path.Width w of central conductor 24 and spacing g between central conductor24 and outer conductors 22 are correspondingly smaller in the shortercoplanar line section 32 as compared to the longer coplanar line section34, in order to obtain the same impedance and ohmic damping. Forexample, as shown in FIG. 4 a, the shorter coplanar conductive path 32or, as shown in FIG. 4 b, the longer coplanar conductive path 34, candeviate from the line geometry that prevails in the remaining coplanarlines, or both of them deviate from a third line geometry that is usedin the rest of the circuit. To prevent reflectances and emissions, thetransitions between the line geometries are designed to be gradual,quasi flowing, over a sufficient length.

Switches 38 that are located at the input and output of phase shifter 30allow the selection of which of the two conductive paths 32, 34 and,therefore, which phase shift, to utilize. Switches 38 are MEM switches.Other switches can also be provided, such as pin diodes, FETs or HEMTswitches.

For use in phased arrays with beam sweeping, for example, detour phaseshifter 30 is inserted in a high-frequency electrical line 36, e.g., infront of an antenna element 9 of a phased array, as shown in FIG. 1. Itis connected at its input and output, in an impedance-adjusted manner,with the ends of high-frequency line 36.

FIG. 5 is a schematic sketch of a taper 40 used in a further developmentof the invention. The line dimensions of central conductor 44, such aswidth w of central conductor 24, and widths ba and bb of outerconductors 22, and widths ga and gb of gaps 26 between lines 22, 24 arechanged with regard for coplanar line sections 46 that are adjacent totaper 40. The ratio of the line dimensions is always selected in such amanner that the line impedance remains the same. Transitions 42 to theline geometries of adjacent coplanar line sections 46 take place viagradual, quasi flowing changes in the line dimensions. As shown in FIGS.5 and 5 a, width w and spacing g (and ga and gb), for example, becomesmaller toward the center of taper 40, whereby the variation sketched inFIG. 5 a is unusual in that it does not have a middle section. Due tothe narrowing of the central conductor, it serves as damping element.

Bridge connections 50 and their application in embodiments according tothe invention are shown in FIGS. 6 through 8.

FIG. 6 is a schematic illustration of a coplanar line with a bridgeconnection 50 shown in cross section from the front. Bridge connection50 is a conductive wafer, composed, e.g., of aluminum, which is attachedto outer conductors 22 and joins them in a conductive manner. In thiscase, outer conductors 22 are higher than central conductor 24, so thatbridge connection 50 has a corresponding spacing from central conductor24. Various other possibilities for bridge connections 50 are alsofeasible, however, to cross central conductor 24 without a conductiveconnection. For example, a connection of outer conductors 22 could runthrough a hidden bridge 50 under central conductor 24, or centralconductor 24 could extend over or tunnel under bridge connection 50. Inintegrated phase shifters (e.g., in MMICs) in GaAs—, SiGe orsilicon/MEMS technology, the bridge is typically formed out of a metallayer that otherwise also covers all lines. The central conductor in thearea of the bridge is composed of a metal layer having a lower height.

FIG. 7 shows a coplanar line section with bridge connection 50 andinductive line section 52 that compensates for its capacitance withregard for impedance. Bridge connection 50 having width A is located inthe center of inductive line section 52. To increase inductivity, linesection 52 has a tapered (narrower) central conductor 24 and outerconductors 22 that are removed therefrom via a larger spacing g and arealso narrower, whereby their width can also be unchanged. Length L ofinductive line section 52 is tailored exactly in such a manner that acompensation of capacitance takes place via bridge connection 50 withregard for impedance. The ohmic damping is increased by the narrowercentral conductor 24. The bridge does not necessarily have to be locatedexactly in the center of the compensating line section.

The ohmic damping of shorter coplanar line 32, as shown in FIG. 8, cantherefore be adjusted to the ohmic damping of longer coplanar line 34 inaccordance with the invention by using bridge connections 50 that arewider and, as a result, equipped with greater capacitance and,therefore, correspondingly longer inductive line sections 52. Bridgeconnections 50 are located on each of the line ends of a coplanar linebranching with MEM switch 38 at the input and output of a detour phaseshifter 30 according to the invention. As a result, the second mode thatinterferes with the signal is optimally suppressed.

Although the present invention was described hereinabove with referenceto a preferred exemplary embodiment, it is not limited thereto; instead,it is capable of being modified in a diverse manner.

For example, phase shifters are also capable of being used that arecomposed of a combination of detour phase shifters, in accordance withthe invention, with another, e.g., “stub-loaded line”, phase shifter.

The phase-shift range can therefore be increased as a result, or a moredetained phase adaptation can take place, whereby the insertion loss canbe kept nearly constant, independently of the phase state, via thetailored sizing of the particular, various-length coplanar lines of thedetour phase shifter.

In addition to its application for sensors in the automotive industry,the phase shifter, according to the invention, can also be used, amongother things, in communication technology for future communication,mobile radio, and satellite radio applications with space-divisionmultiple access (SDMA: user connections over spacially limited,user-specific radiation lobes of the base station or satellite and/orthe user unit), and civil or military radar systems.

Finally, features of the subclaims can be essentially combined freelywith each other, and not in the order in which they appear in theclaims, as long as they are independent of each other.

Literature

-   [1] N. Fourikis, Advanced Array Systems, Applications and RF    Technologies, Academic Press, San Diego, etc., 2001-   [2] R. i. Maillous, Phased Array Antenna Handbook, Artech House,    Boston, London 1994.-   [3] D. M. Pozar, D. H. Schaubert, Microstrip Antennas, IEEE Press,    New York 1995.-   [4] S. K. Koul, B. Bhat, Microwave and Millimeter Wave Phase    Shifters, Vol. 1 and 2, Artech House, Boston, London 1991.-   [5] R. K. Hoffmann, Integrierted Mikrowellenschaltungen,    Springer-Verlag, Berlin, etc., 1983.-   [6] G. Ghione, C. U. Naldi, Coplanar Waveguides for MMIC    Applications: Effect of Upper Shielding, Conductor Backing,    Finite-Extent Ground Planes, and Line-to-Line Coupling, IEEE Trans.    Microwave Theory Tech. MTT-35, 260-267, 1987.-   [7] G. Ghione, A CAD-Oriented Analytical Model for the Losses of    General Asymmetric Coplanar Lines in Hybrid and Monolythic MICS,    IEEE Trans. Microwave Theory Tech. 41, 1499-1510, 1993.-   [8] A. Malczweski, S. Eschelman, B. Pillans, J. Ehmke, C. L.    Goldsmith, X-Band RF MEMS Phase Shifters for Phased Array    Applications, IEEE Microwave Guided Wave Lett. 9, 517-519, 1999.-   [9] B. Pillans, S. Eshelman, A. Malczewski, J. Ehmke, C. L.    Goldsmith, KA-Band RF MEMS Phase Shifters for Phased Array    Applications, IEEE MTT-S International Microwave Symposium Digest,    IEEE, New York, 2000.-   [10] K. Zuefle, F. Steinhagen, W. H. Haydl, A. Hülsmann, Coplanar    4-bit HEMT phase shifters for 94 GHz phased array radar systems,    IEEE MTT-S International Microwave Symposium Digest, IEEE, New York,    1999.-   [11] E. Rius, J. P. Coupez, S. Toutain, C. Person, P. Legaud,    Theoretical and Experimental Study of Various Types of Compensated    Dielectric Bridges for Millimeter-Wave Coplanar Applications, IEEE    Trans. Microwave Theory Tech. 48,152-156, 2000.-   [12] R. E. Collin, Foundations for Microwave Engineering, 2^(nd) ed.    McGraw-Hill, New York, etc., 1992.

1. A device for phase shifting for high-frequency electric lines (36),whereby phase shifting is essentially achieved by specifically selectingthe line length, wherein a circuit arrangement (30) equipped withcoplanar lines (10) is provided, the adjustment possibilities of whichsaid various-length coplanar lines with regard to ohmic damping andimpedance are preselected in such a way that ohmic damping and impedanceare essentially the same on the selectively controllable, various-lengthconductive paths (32, 34) of the circuit arrangement (30).
 2. The deviceas recited in claim 1, wherein the adjustments with regard for ohmicdamping and impedance for the various-length coplanar conductive paths(32; 34) are provided at the least via a specifically preselected widthw of the central conductor (24) and a specifically preselected spacing gof the central conductor (24) from the outer conductors (22).
 3. Thedevice as recited in claim 2, wherein the width b of the outerconductors (22) of the various-length, coplanar conductive paths (32;34) is specifically preselected.
 4. The device as recited in claim 1,wherein at least one coplanar line (10) of the various-length conductivepaths (32, 34) contains at least one taper (40).
 5. The device asrecited in claim 1, wherein at least one conductive bridge connection(40) is located between each of the outer conductors (22) of eachcoplanar conductive path (32, 34).
 6. The device as recited in claim 5,wherein, for line branchings, the bridge connections (50) are located atleast on each of the branching-in and branching-off areas of thecoplanar lines (10).
 7. The device as recited in claim 5, wherein theparticular coplanar conductive paths (32, 34) contain at least oneinductive line section (52) that is designed to compensate for theadditional capacitance, with regard for line impedance, that is broughtabout by the bridge connections (50).
 8. The device as recited in claim7, wherein the various-length, coplanar conductive paths (32, 34) forohmic damping that is essentially the same overall include inductiveline sections (52) that differ in terms of the width and length of atapered (narrower) central conductor (24), whereby the particular bridgeconnections (50) are configured in terms of shape and/or type to bringabout the particular different, compensating capacitance with regard forline impedance.
 9. The device as recited in claim 8, wherein theparticular compensating capacitance is brought about using various-widthbridge connections (50).
 10. The device as recited in claim 7, whereinthe various-length, coplanar conductive paths (32; 34), for damping thatis the same overall, contain a different number of identical inductiveline sections (52) with a tapered central conductor (24), whereby thebridge connections (50) have an identical configuration to bring aboutthe particular compensating capacitance.
 11. The device as recited inclaim 1, wherein, for ohmic damping that is essentially the sameoverall, damping material with correspondingly high additional ohmicdamping is applied on the coplanar lines of the conductive paths (32)that are shorter than the longest conductive path (34).
 12. The deviceas recited in claim 1, wherein the sizes of the cross sections of thecentral conductors (24), in particular with regard for the height of thecentral conductors (24), with consideration for additional ohmicdampings that are induced by bends in the line in particular, aredesigned for the particular, various-length coplanar conductive paths(32, 34) in such a manner that the ohmic damping on the conductive pathsis essentially the same.
 13. The device as recited in claim 1, wherein,for the ohmic damping of the various-length conductive paths (32, 34)that is essentially the same overall, the central conductors (24) of theshorter conductive paths (32) are composed of a material havingcorrespondingly lower conductivity.
 14. The device as recited in claim1, wherein, for the damping that is essentially the same overall, theconductivity of the substrate (20) of the particular coplanar conductivepaths (32, 34) is designed differently accordingly.
 15. The device asrecited in claim 1, wherein a layer, composed of silicon oxide inparticular, is inserted—along a length that is adjusted accordingly—inthe gaps (26) between the central conductor (24) and the outerconductors (22) for the damping—that is essentially the same overall—ofthe coplanar lines, each having various-length conductive paths (32,34).
 16. The device as recited in claim 1that containsmicroelectromechanical switches (MEM switches) (38) for switching over.17. Phased arrays (1) containing a device as recited in claim 1.